The present invention relates to a process for the direct joining of rubber to metal surfaces.
A good bond of natural or synthetic rubber to metal is desirable for numerous purposes, but especially in the manufacture of vehicle tires with metal cord inserts. In this case, it is important that the bond between the metal and the rubber has as high an adhesive strength as possible, as well as good elasticity and long life. Only thus can optimum running properties and in particular the safety of vehicle tires at high speed be achieved. Insufficient adhesive strength and elasticity of the metal/rubber bond lead to early destruction of the tire body.
Aids in the adhesion of rubber mixtures to metal surfaces have included sulfur, proteins such as albumin, resorcinol-formaldehyde, poly-isocyanantes, heavy metal salts of organic acids, calcined heavy metal salts of organic acids, iron oxide 2-(2,4-dinitrophenylthio)-benzothiazole, heavy metal salts of hydroxy-benzoic acid as well as Co-napthenates.
The use of tellurium, tellurium alloys of tellurium compounds as bonding aids is not known in the prior art.
In Italian Pat. No. 378,711, a process is described wherein rubber and metal are durably joined through an intermediate layer. Applying an intermediate layer on the metal cord, however, involves increased technical cost as well as increased investments and production costs in tire manufacture and moreover constitutes an additional source for error or fault.
In French Pat. No. 960,629, a process for achieving good adhesive strength between metal and rubber mixtures is described wherein very high proportions of heavy metal compounds and sulfur are used in the rubber mixture. With this process, however, a hard rubber is formed which is not suitable for vehicle tires.
In DE-AS No. 1 130 159, a process for improving the adhesive strength of metal to rubber is described wherein a vulcanizable rubber mixture is vulcanized to a metal in the presence of a heavy metal salt of an organic acid, the heavy metal salt being used in a partially calcined or oxidized state. The disadvantage of this process is that the calcining process increases the cost of these additives so that their use in the mass production of tires is uneconomical. Besides, the adhesive strengths obtained with this process are not sufficient and in particular not for tires which are to be used at high speed.
In DE-OS No. 26 16 613, it is shown that, by the addition of metal salts of hydroxybenzoic acid to a rubber composition, the latter's adhesion to brass-plated steel cord is improved after the vulcanization; and the impairment of the adhesive properties by heat aging is reduced. However, when the rubber composition described in DE-OS No. 26 16 613 is used for making steel cord tires, it is found that the adhesion is insufficient and that it becomes worse with heat aging.
In Ullmann's Enzyklopedie der Technishen Chemie (Encyclopedia of Technical Chemistry), 9th Volume, Urban and Schwarzenberg, Munich-Berlin 1957, there is described on page 382, vulcanizing agents for cold vulcanization and for hot vulcanization. Under cold vulcanization, it is stated that the use of tellurium chloride among other compounds for the vulcanization of rubber instead of sulfur chloride has not been adopted in the practice. Under hot vulcanization, it is stated that also selenium and tellurium have been used for the vulcanization of rubber, but that the effect of these products is weaker than that of sulfur. On page 385, it is further stated that the tellurium salts of N-dialkyl-N-alkylaryl dithiocarbamic acids can be used as organic accelerators, but that these salts are little used in technical practice. There are no indications of any kind that tellurium, tellurium alloys or tellurium compounds can serve as means for bond improvement between metal surfaces and rubber.
It is the object of the present invention to provide a process for joining rubber mixtures to metal surfaces, in particular to metal cord, which does not have the disadvantages of the known processes and with which in particular better adhesive strengths between rubber and metal are obtained. Furthermore, the process of the invention is well reproducible and provides a minimum of error or fault sources, as well as low investment and production costs.